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Caption

Computer illustration of a T4 bacteriophage virus. The head contains DNA (deoxyribonucleic acid)) inside a protein coat. Attached to this is the tail, consisting of a tube-like sheath and tail fibres (at bottom). T4 bacteriophages are parasites of Escherichia coli, a bacterium common in the human gut. The virus attaches itself to the host bacterium cell wall by its tail fibres; the sheath then contracts, injecting the contents of the head (DNA) into the host. The viral DNA makes the bacteria manufacture more copies of the virus.